Magnatone Custom 260 Amp...weak Vibrato

[nevetslab]

This is the last of the three Magnatone amps I've been going thru and restoring for my client. This one has lots of battle scars physically, and, as far as how it sounds....less than stellar. I just got thru pulling the chassis from the cabinet to see what's inside. Easier to work on then the older Fiberglass housed amp assemblies, though the M10A I just restored rocks....this one.......maybe weak tubes? I know I don't have any 6CG7 tubes that's used in the Vibrato circuit. Maybe substitute a couple 12AU7's in their place?

There's a shock-mount platform that the tube circuits ride on....though looks like the rubber suspension parts are worn out.



I see replacement pots and Sprague Orange Drop caps installed, date codes late 1990's to early 2000's. Power cord was replaced, it now being a 3-wire grounded cable, though the ground connection was made to one of the power transformer core bolts, and the death cap was left in place between the amp-side of the fuse holder and the Ground wire of the Standby Switch (S/B achieved by opening the HT C/T). I'll correct those issues.





There appears to be an added Slow Speed Vibrato mod added, not shown on the schematic. I've attached the Custom 260 schematic below:

magnatone_260a Schematic.pdf

[nevetslab]

I spent the day with this amp. First, I added 22uF/50V Cathode Bypass caps onto the 470 ohm cathode resistors of the input stages. That helped considerably, now having gain again, though only on Ch 1. There was NO Bass Response on either channel's bass controls. I found on Ch 1, a 22nF Orange Drop placed across the 4.7nF cap on the Bass circuit, with it's partner being a 470pF. Ch 2 had an old 47nF placed across that same location on the pot, but.....when the controls were replaced previously, the tech mixed up the 470pF and 4.7nF disc caps, so I had to restore that placement. With those two changes, now the Bass Control worked on both channels. Ch 1 had way more gain than Ch 2, due to the 12AX7 stage following the Ch 1 Volume control. Ch 2 didn't have that advantage, and instead was mixed with the output from the vibrato circuit. I tried summing the two volume pots with 270ks ahead of the second gain stage, and then changed the output of the vibrato circuit, adding a 1M bias resistor for the input grid of that 12AX7, while lowering the 10M to 1M. While that did give more gain to Ch 2, and both channels now had Vibrato, it also added a lot of hum that wasn't there before. So, I restored the gain structure as it was, leaving Ch 2 with considerably lower gain than Ch 1.

The Vibrato still isn't real strong, but is working, and now with plenty of gain and the Bass Controls working, it sounds respectable. Still looks pretty beat up, and little I can do about that. There wasn't the budget to recap the supplies on this, nor the majority of the Cap-can based amps in this collection of vintage amps. Next owner's task. I did at least take the control knobs to the kitchen and give them a good scrubbing, then had to tension the spline shafts of the controls so all the knobs fit snug, as they were falling off. I did cut the death cap off once I got started this morning. Changed the input stage tube, as the original was noisy.

I did try a pair of 12BH7's in place of the 6CG7's....not an exact substitution, but figured I'd give them a try. No change in the Vibrato circuit, so I restored the 6CG7's.

[Leo_Gnardo]

The Vibrato still isn't real strong, but is working

When I encounter failed or weak vibrato, I replace the trio of caps in the oscillator bridge plus the cap that delivers the LFO to the depth control, also the cap that filters higher frequency LFO "noise" to ground. Looks like Mag used the same fairly reliable blue film caps we see in mid 60's Fenders, maybe that's the reason the vibrato remains working at all. Still worth a refresh. I'd recommend either polypro or mylar Panasonic film caps, low cost and highly reliable. The brown "chiclets." For a few more $$ could go with orange drops or Mallory 150's but I doubt improvement would be detected compared to the Panasonics. Any cathode bypass caps in the oscillator circuit ought to be refreshed as well.

Rubber "shock absorbers" in all the old amps have crumbled into dust, turned to stone, or become globs of goo. What a PIA to replace. Those amp manufacturers never dreamed the amps they built would still be used, much less sought after, 50 60 70 years later.